The present invention concerns improved hard-soft ionomeric resin mixtures (or blends) which are particularly well suited for the formulation of the cover composition of a golf ball. In addition, the present invention is directed to the improved low modulus golf balls produced utilizing the cover compositions of the invention.
More specifically, the present invention relates to novel golf ball cover compositions of low modulus blends comprising about 25 to about 10 (preferably from less than 25 to about 15) weight percent of hard ionomer resins and from about 75 to about 90 (preferably from greater than 75 to about 85) weight percent of soft ionomer resins. The new compositions of the present invention, when utilized for golf ball construction, particularly the construction of two piece golf balls, produce golf balls exhibiting enhanced playability (spin rate) characteristics without substantially sacrificing such properties as travel distance, roundness, impact resistance and/or durability.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E.I. DuPont deNemours and Company (see U.S. Pat. No. 4,884,814) under the trademark xe2x80x9cSurlyn(copyright)xe2x80x9d and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks xe2x80x9cEscor(copyright)xe2x80x9d and the tradename xe2x80x9cIotekxe2x80x9d, have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability necessary for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc., for golf ball cover construction over balata. However, the advantages gained in increased durability have been offset to some degree by the decreases produced in playability. This is because although the ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight.
As a result, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the xe2x80x9chardxe2x80x9d ionomeric resins, but also the playability (i.e. xe2x80x9cspinxe2x80x9d) characteristics previously associated with the xe2x80x9csoftxe2x80x9d balata covers, properties which are still desired by the more skilled golfer.
In various attempts to produce such an ideal golf ball, the golfing industry has blended the hard ionomeric resins with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally been dissatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
In addition, various xe2x80x9chard-soft ionomeric blendsxe2x80x9d, i.e. mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. However, until the development of the specific blend combination set forth in U.S. Pat. No. 4,884,814, directed to relatively low modulus golf ball cover compositions, these balls were not particularly commercially viable. In this regard, although the balls produced using the hard-soft ionomer blends exhibited enhanced playability characteristics, they lacked the durability needed for continuous play.
U.S. Pat. No. 4,884,814, one of the inventor""s previous patents, is directed to the finding that if various xe2x80x9chardxe2x80x9d methacrylic acid based ionomer resins (i.e. those ionomer resins having a hardness of about 60 to 66 on the Shore D scale as measured in accordance with ASTM method D-2240) were blended with similar or larger quantities of one or more xe2x80x9csoftxe2x80x9d ionomer methacrylic acid based ionomer resins (i.e. those ionomer resins having a hardness from about 25 to 40 as measured on the Shore D scale) that relatively low modulus golf ball cover compositions could be produced that are not only softer than the prior art hard ionomer covers but also exhibit a sufficient degree of durability for repetitive play. These relatively low modulus cover compositions were generally comprised of from about 25 to about 70 weight percent of hard ionomer resins and from about 30 to about 75 weight percent of soft ionomer resins. The incorporation of larger percentages of soft ionomer resin into the cover formulations was generally deemed to be undesirable due to the sacrifices in distance and/or durability that were produced thereby.
However, notwithstanding the above, it has now been found that specific hard/soft ionomer resin blends comprising from about 25 to about 10 (preferably from less than 25 to about 15) weight percent of one or more hard ionomer resins and from about 75 to about 90 (preferably from greater than 75 to about 85) weight percent of one or more soft ionomer resins offer, when utilized for golf ball cover construction, particular advantages to the high skilled professional golfer. Specifically, it has been found that golf balls produced using the new low modulus cover compositions of the invention exhibit high spin rates at very low club head speeds. These balls offer particular utility for the highly skilled and/or professional golfer who desires greater control on the low club head speed xe2x80x9cpitchxe2x80x9d or xe2x80x9cwedgexe2x80x9d shots.
In the past, covers composed of high levels of soft ionomers were considered to be nonfunctional due to the drop off in C.O.R. (i.e. travel distance) and cut resistance that accompanied the increase in softness. However, for the P.G.A. Touring Professional Golfer and other highly skilled golfers, distance and durability are generally not a concern. Along these lines, the P.G.A. Touring Professional Golfer will almost never mis-hit a ball badly enough to cut the cover. In addition, the club head speed generated by professional golfers is typically much higher than the amateur, thereby producing greater distance. Thus, the resilience characteristic of the ball is of less importance for the professional golfer than for the average player.
Moreover, the P.G.A. Touring Professional Golfer is usually willing to sacrifice distance for added control, particularly with respect to shots around the green. It is in these pitch, or half and quarter wedge shots that the balls of the present invention show a distinct advantage over the hard/soft ionomer blends of the prior art and the balata covered wound balls.
The present invention is directed to new golf ball cover compositions which exhibit properties of enhanced playability characteristics (i.e. softness and spin) over known hard-soft ionomer blends and the balata-covered wound balls. It has been found that these properties can be produced using improved hard-soft ionomer blends of the present invention in order to produce a low modulus golf ball cover.
The foregoing has outlined some of the most pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention, the detailed description of the invention and the claims which follow below.
The present invention is directed to improved cover compositions for golf ball construction and the resulting golf balls produced utilizing the improved cover compositions. The novel golf ball cover compositions of the invention comprise a blend of hard ionomeric resins, preferably acrylic acid based ionomers, and recently developed acrylic acid based soft ionomers. When the cover compositions of the invention are utilized to manufacture golf balls, the golf balls produced thereby, exhibit properties of improved distance without sacrificing playability and/or durability when compared to known hard-soft ionomer blends.
The novel golf ball cover compositions of the invention comprise a blend of about 25 to about 10 (preferably from less than 25 to about 15) weight percent of hard ionomeric resins and from about 75 to about 90 (preferably from about 10 to greater than 75) of soft ionomer resins. In the more preferred embodiment of the invention, the soft ionomer resins are methacrylic and acrylic acid based soft ionomer resins. Most preferably, the soft ionomer resins are acrylic acid based soft ionomers. When the cover compositions of the invention are utilized to manufacture golf balls, the golf balls produced thereby exhibit properties of improved playability characteristics, particularly at low swing speeds, without producing substantial sacrifices in distance and/or durability when compared to known hard-soft ionomer blends.
Two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (C.O.R.), the constant xe2x80x9cexe2x80x9d, which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact. As a result, the coefficient of restitution (i.e. xe2x80x9cexe2x80x9d) can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision.
Resilience (C.O.R.), along with additional factors such as club head speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determines the distance a ball will travel when hit. Since club head speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e. balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings. Although both the core and the cover contribute to the coefficient of restitution, the present invention is directed solely to the coefficient of restitution which is affected by the cover composition.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball""s incoming and outgoing velocity electronically. As mentioned above, the coefficient of restitution is the ratio of the outgoing velocity to incoming velocity. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golf Association (U.S.G.A.). Along this line, the U.S.G.A. standards indicate that a xe2x80x9cregulationxe2x80x9d ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second. Since the coefficient of restitution of a ball is related to the ball""s initial velocity, it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of softness (i.e. hardness) to produce enhanced playability (i.e. spin, etc.).
The hardness of the ball is the second principal property involved in the performance of a golf ball. The hardness of the ball can affect the playability of the ball on striking and the sound or xe2x80x9cclickxe2x80x9d produced. Hardness is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball""s diameter (i.e. the lower the compression value, the harder the material). The xe2x80x9csofterxe2x80x9d covers permit the accomplished golfer to impart proper spin. This is because the softer covers deform on impact significantly more than balls having xe2x80x9charderxe2x80x9d ionomeric resin covers. As a result, this allows the better player to impart fade, draw, or backspin to the ball thereby enhancing playability. Such properties can be determined by various xe2x80x9cspin rate testsxe2x80x9d, such as the xe2x80x9cnine-ironxe2x80x9d spin rate test set forth below.
Accordingly, the present invention is generally directed to new hard-soft ionomer blends which produce, upon molding around solid or wound cores to formulate a low modulus cover composition, golf balls exhibiting enhanced playability (i.e. hardness/softness, spin rates, etc.) properties without substantially adversely affecting the ball""s durability (i.e. impact resistance, etc.) and distance characteristics when utilized by the more skilled or professional golfer. The golf balls which produce high spin rates even at very low club head speeds have particular utility for the professional golfer who desires greater control or xe2x80x9cpitchxe2x80x9d or xe2x80x9cwedgexe2x80x9d shots.
These and other objects and features of the invention will be apparent from the following description and from the claims.